The invention relates to electromagnetic wave shielding circuits and production thereof, wherein the electromagnetic wave shielding effect is excellent and the shielding circuits are securely fixed to a base board, and moreover the practical use may be highly appreciated.
It is generally known that the various electric or electronic instruments will produce electromagnetic waves to give an adverse influence such as the electromagnetic interference to other electric or electronic instruments. In fact, the legal regulation of such electromagnetic interference has been recently severe in many countries. The electromagnetic waves are such waves as are flowing in the space around us and are widely commercialized in the range of broadcast from a low frequency of the radio electromagnetic waves to an extremely high frequency of the space satellites. Such commercialized electromagnetic waves around us are legally regulated so as not to cause the electromagnetic interferences. There are, however, some instruments which may inevitably radiate or leak considerably strong electromagnetic waves. Such instruments are generally required to have means provided for shielding the electromagnetic waves as described, for example, in a Japanese periodical Function Materials, June issue 1984, pages 22-23.
Recently various function materials have been proposed for shielding the electromagnetic waves. These function materials are, however, not so effective as the widely employed metal plate enclosure shielding with respect to the usability and endurance Other materials proposed are not preferable, for example, such as a ceramics of ferrite, a plastic of polystyrene, a polystyrene containing copper filler an electrically conductive coating material to be flame-coated, vacuum-deposited, spattered or reduction-treated, a metal foil to be adhered, a resin to be coated, a metal to be meshed, etc.. For example, the polystyrene represents at most 15 dB against the electromagnetic waves of frequency 600 MHz, which is the minimum effect for shielding the electromagnetic waves. Concerning the polystyrene containing copper filler, the shield effect is best, that is, about 55 dB at about the frequency 500 MHz, and the shield effect is at most 25 dB at the frequency 1 kHz and 1 GHz.
The mentioned Function Materials, June issue, 1984, P. 24 indicates the shield effect (SE) in the unit of decibel (dB) as follows; ##EQU1## the levels of shield effect (SE) are by way of experience, 0 dB .about.10 dB : almost no shield effect may be confirmed,
10 dB .about.30 dB : minimum shield effect may be confirmed, PA0 30 dB .about.60 dB : average shield effect may be confirmed, PA0 60 dB .about.90 dB : considerably high shield effect may be confirmed, PA0 over 90 dB : shield effect by way of a best technique.
In compliance to the legal regulations in many countries, the materials with a shield effect of about 40 dB .about.50 dB may be practically used.
The inventors asked the Tokyo Metropolitan Industrial Center to measure the shield effect (SE) of the various function materials as shown in FIG. 9 by way of the coaxial transmission line method as shown in FIG. 7. As the result, the carbon paste (20 .OMEGA./.quadrature.) is about 17 dB which is stabilized against the frequency F; the nickel paste (10.sup.-2 .OMEGA.-cm) is about 26 dB .about.28 dB which is a little lower against the frequencies of more than 700 MHz; the silver and carbon paste (1.0 .OMEGA./.quadrature.) is about 36 dB which is stabilized against the frequency F; the copper paste (0.1 .OMEGA./.quadrature.) is of a shield effect which is lower as the frequency F increases, that is, the shield effect is about 58 dB against the frequency 30 MHz, which is of a desired value, but is lower down to 43 dB against 1000 MHz. As to the aluminium foil, though not shown, the shield effect of about 45 dB .about.65 dB may be obtained, and it is confirmed that the shield effect will increase up to about 60 dB as the frequency F increases. But such a value is still not sufficient.
Japanese utility model application published as 55-4246 discloses an electromagnetic wave shield by way of making copper plating on a coating having electrically conductive copper particles. According to this method, it is required that the coating containing electrically conductive copper particles has a high electric conductivity, and has a phenol resin contained therein as a binder.
However according to the experiments made by the present inventors, 10% by weight of phenol resin against 100% by weight of copper particles will make the coating practically useless because the coating is dissolved into the plating solution, and further 20% by weight of phenol resin is not enough to secure a sufficient adherence force to the copper foil after plating and to the base material. In any event, it is apparent that the phenol resin is practically useless to be used as a binder as shown in Table 1. There are no other electromagnetic wave shielding techniques proposed for realizing an excellent shielding effect by making a copper plating on the coating containing copper particles, in which the binder is effective to sufficiently adhere to the copper foil and to the base material in the initial stage of plating and after thereof.